MAPPING OF THYROTROPIN-RELEASING-HORMONE (TRH) NEURONAL SYSTEMS OF RAT FOREBRAIN PROJECTING TO THE MEDIAN-EMINENCE AND THE OVLT - IMMUNOCYTOCHEMISTRY COMBINED WITH RETROGRADE LABELING AT THE LIGHT AND ELECTRON-MICROSCOPIC LEVELS

The thyrotropin-releasing hormone (TRH)-containing neurons that projec t to portal capillaries of the external zone of the median eminence (M E) and fenestrated capillaries of the organum vasculosum of the lamina terminalis (OVLT) were identified on thin paraffin and thick vibratom e sections using a combination of retrograde labeling with peripherall y administered Fluoro-Gold and fluorescence immunocytochemistry. The r esults indicate that the vast majority of those TRH neurons that proje ct to the ME and the OVLT is located in the paraventricular nucleus (P VN), and most abundantly, in its medial parvicellular subdivision. Alt hough numerous TRH-immunoreactive (TRH-i) neurons are present in other hypothalamic areas of the brain, only a few of them in the dorsal hyp othalamic area behind the PVN and the periventricular preoptic nucleus could be retrogradely labeled. Since only a few Fluoro-Gold-accumulat ing and TRH-i perikarya were seen in other nuclei than the PVN, it is likely that the majority of nerve terminals in the OVLT also originate s from TRH-i perikarya in the PVN. Fluoro-Gold, an electron-dense subs tance, is stored in the lysosomes of hypophysiotropic TRH-i perikarya and thus, it provides an excellent model for electron microscopic char acterization of hypophysiotropic neurons at both the light and electro n microscopic levels. The data together provides additional morphologi cal evidence for the key role of the PVN in the regulation of TSH secr etion.